1. Technical Field
The present invention relates to a micro resonator sensor, and more specifically, to a sensor for detecting a characteristic of a material to be measured (also called “to-be-measured material”) using changes in a refractive index of a resonator.
2. Related Art
Generally, a resonator sensor serves to detect a characteristic of a measured-material by detecting intensity of light at the output terminal of a waveguide that is formed with an input terminal and an output terminal. In this case, the intensity of light corresponds to a change of an effective refractive index of a ring resonator that occurs when light traveling through the waveguide is coupled to the ring resonator installed to be spaced apart from the waveguide.
FIG. 1 is a view showing a conventional micro ring resonator sensor.
Referring to FIG. 1, a conventional micro ring resonator comprises a main waveguide 110 and a ring resonator 120. The main waveguide 110 is constructed with an optical fiber or an optical waveguide, and both ends of the main waveguide 110 respectively function as an input terminal for receiving optical signals and an output terminal for outputting optical signals. The ring resonator 120 is an optical fiber or an optical waveguide of a ring shape having a predetermined radius R, and the ring resonator 120 has an opening 122 whose surface is interface-processed so that light passing through the optical fiber or the optical waveguide forming the ring resonator 120 may effectively react with liquid or gas, which is a measured-material. The opening 122 is formed on the top or a side surface of the optical fiber or the optical waveguide configuring the ring resonator 120. An optical transfer mode that can be accepted by the micro ring resonator sensor is determined depending on the position of the opening 122. Accordingly, if the opening 122 is formed on both of the top and side surfaces of the ring resonator 120, optical signals of both TM and TE modes can be received. The main waveguide 110 and the ring resonator 120 are arranged on one dielectric substrate to be spaced apart from each other to configure a ring resonator sensor.
As shown in FIG. 1, in the conventional micro ring resonator sensor, an optical signal inputted through the input terminal of the main waveguide 110 advances along the main waveguide 110 and is coupled to the ring resonator 120 depending on a resonance condition of the ring resonator 120 that is arranged to be spaced apart from the main waveguide 110. At this point, the light inputted into the ring resonator 120 reacts with a bio-material in a liquid or gaseous state, which is a measured-material, on the interface-processed surface of the opening 122 formed at the ring resonator 120, and thus the effective refractive index of the ring resonator 120 is changed. Then, as the effective refractive index of the ring resonator 120 is changed, a condition for optical coupling from the main waveguide 110 to the ring resonator 120 is changed. At this point, the effective refractive index of the ring resonator 120 is changed in correspondence with concentration of the material reacting on the top and side surfaces of the ring resonator 120. Accordingly, the amount of light outputted through the output terminal of the main waveguide 110 is changed, and thus the characteristic of the material can be detected. In this manner, if a bio-transducer is configured by employing a biological element at the opening 122 of the ring resonator 120, a bio-sensor using a ring resonator can be manufactured.
Since any reflection does not occur inside of the ring resonator 120 of the convention micro ring resonator sensor having four ports a1, a2, a3, and a4 shown in FIG. 1, the initial condition is b1=b3=a2=a4=zero. Accordingly, the characteristic function of the micro ring resonator sensor shown in FIG. 1 is expressed as shown in the following Equation.
                                                      J                    ⁢                                                                      b                  1                                                                                                      b                  2                                                                                                      b                  3                                                                                                      b                  4                                                              ⁢                                  K                          =                                                          J                        ⁢                                                            0                                                                                            1                      -                                              k                        2                                                                                                              0                                                  jk                                                                                                                        1                      -                                              k                        2                                                                                                              0                                                  jk                                                  0                                                                              0                                                  jk                                                  0                                                                                            1                      -                                              k                        2                                                                                                                                          jk                                                  0                                                                                            1                      -                                              k                        2                                                                                                              0                                                      ⁢                                        K                                ⁢                                                  J                        ⁢                                                                                a                    1                                                                                                                    a                    2                                                                                                                    a                    3                                                                                                                    a                    4                                                                        ⁢                                        K                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, |k2| is intensity of an optical signal coupled from port 1 a1 to port 4 b4 when the light signal passes through the optical waveguide once, and |1−k2| is intensity of an optical signal passing through without being coupled when the light signal passes through the optical waveguide once.
In addition, the optical signal inside of the ring resonator can be expressed in the following Equation.a3=b4e−(αR+jφR)  [Equation 2]
In Equation 2, αR is a loss occurred when the optical signal passes through inside of the ring resonator once, and φR is a phase difference occurred when the optical signal passes through inside of the ring resonator once.
On the other hand, the following expression can be obtained from Equation 1.b1=√{square root over (1−k2)}a2+jka4 b2=√{square root over (1−k2)}a1+jka3 b3=jka2+√{square root over (1−k2)}a4 b4=jka1+√{square root over (1−k2)}a3  [Equation 3]
In addition, the following expression is derived from Equations 2 and 3.
                              b          2                =                              [                                                                                1                    -                                          k                      2                                                                      -                                  ⅇ                                      -                                          (                                                                        α                          R                                                +                                                  j                          ⁢                                                                                                          ⁢                                                      ϕ                            R                                                                                              )                                                                                                  1                -                                                                            1                      -                                              k                        2                                                                              ⁢                                      ⅇ                                          -                                              (                                                                              α                            R                                                    +                                                      j                            ⁢                                                                                                                  ⁢                                                          ϕ                              R                                                                                                      )                                                                                                                  ]                    ⁢                      a            1                                              [                  Equation          ⁢                                          ⁢          4                ]            
Then, resonance of the ring occurs when φ=2mπ in Equation 4, and at this point, Equation 4 is rearranged as shown below.
                                          b            2                                a            1                          =                              [                                                                                1                    -                                          k                      2                                                                      -                                  ⅇ                                      -                                          α                      R                                                                                                  1                -                                                                            1                      -                                              k                        2                                                                              ⁢                                      ⅇ                                          -                                              α                        R                                                                                                                  ]                    ⁢                      a            1                                              [                  Equation          ⁢                                          ⁢          5                ]            
If a resonance condition defined as shown in Equation 5 occurs, a coupling occurs from the main waveguide 110 to the ring resonator 120, and since a critical coupling condition is satisfied when a condition such as the Equation shown below is satisfied, an optical signal is not outputted to the output terminal of the main waveguide 110. At the time point when the critical coupling condition is satisfied, intensity of the optical signal coupled from the main waveguide 110 to the ring resonator 120 becomes the maximum.√{square root over (1−k2)}=e−αR  [Equation 6]
In a resonance state, the critical coupling condition is determined by adjusting the coupling coefficient k and the loss coefficient αR. At this point, the coupling coefficient k is determined by a distance spaced between the main waveguide 110 and the ring resonator 120, and the loss coefficient αR is determined by a reaction of the optical signal and the bio-material at the opening 122 formed at the ring resonator 120.
FIG. 2 is a view showing an example of a characteristic curve of an output light corresponding to the wavelength of an incident light in accordance with a resonance condition of the ring resonator 20 when an optical signal is inputted into the main waveguide 110.
Referring to FIG. 2, if a critical coupling occurs under the resonance condition of the ring resonator 120, an output is not occurred at the output terminal of the main waveguide 110 at the minimum wavelength, and here, the minimum wavelength is moved by interactions among bio-molecules. That is, the wavelength of an optical signal at which an output is not occurred at the output terminal of the main waveguide 110 is changed in accordance with the variation of the effective refractive index of the ring resonator 120 invited by a measured-material contacting with the opening 122 of the ring resonator 120.
Referring to FIG. 2, it is understood that whenever the effective refractive index of the ring resonator 120 is increased by 1×10−4, the minimum wavelength at which an output is not occurred at the output terminal of the main waveguide 110 is constantly increased. Accordingly, the ring resonator sensor can detect a characteristic of a measured-material by detecting a response signal for the intensity and wavelength of an optical signal outputted through the output terminal of the main waveguide 110.
On the other hand, the output of the ring resonator sensor is very sensitive to the change of the dielectric constant of a medium that occurs when the medium is in contact with the opening 122 formed at the ring resonator 120. That is, the dielectric constant of the medium changes as the medium flows through the opening 122 of the ring resonator sensor, and accordingly, the effective refractive index of the ring resonator 120 is changed. Such a change of the effective refractive index of the ring resonator 120 invites a change in a resonance condition, and thus the wavelength of the output signal is moved. Accordingly, the ring resonator sensor detects a characteristic of a measured-material by grasping concentration of the measured-material through the effective refractive index of the ring resonator 120 calculated based on the intensity and phase of the optical signal measured at the output terminal of the main waveguide 110.
The ring resonator sensor described above can be implemented in the form of a bio-sensor, in which while a bio-molecule among bio-molecules combined to each other is fixed on the surface of the opening 122 formed at the ring resonator sensor, a bio-molecule corresponding to the fixed bio-molecule as a measured-material is in contact with the surface of the opening 122, and then a bonding activity between them is detected. Examples of the bio-molecules bonding to each other include antibody-antigen, hormone-receptor, protein-protein, DNA-DNA, DNA-protein, and the like. The bio-sensor that uses a ring resonator like this is a sensor where a ligand is fixated on the surface of the opening 122 of the ring resonator sensor. A method of chemically adsorbing a thiolized ligand on a metal surface can be an example of a method of fixating the ligand, in which the ligand is thiolized by bonding a thiol group to the ligand through a covalent bond. In addition, there also is a method of fixating the ligand on the surface of the opening 122 of the ring resonator sensor using a hydrogel matrix configured in a carboxyl-methylated dextran chain. Such a ring resonator sensor is most advantageous in that a molecule can be directly measured without using an indicator material, such as a radioactive material or a fluorescent material. Furthermore, if a ring resonator bio-sensor is used, a process of bonding bio-molecules can be monitored in real-time.
However, although the ring resonator sensor is advantageous in that a characteristic of a measured-material can be measured with a simple configuration, there is a certain limit in the aspect of miniaturizing the sensor. That is, in the case of a conventional ring resonator sensor provided with a resonator where a waveguide is formed in a loop shape of a circular form, it needs to deeply etch a neighborhood of an optical waveguide configuring the ring resonator in order to reduce the radius of the ring resonator without an excessive radiation loss. If the neighborhood of the optical waveguide configuring the ring resonator is deeply etched, although the effect of optical confinement on the side surface of the optical waveguide can be enhanced, there is a problem in that the optical propagation loss is increased due to sidewall roughness. In addition, if the optical waveguide forming the ring resonator is made of an intrinsic material, etching through the intrinsic material brings about a problem appeared due to excessive surface recombination. Furthermore, such a ring resonator invites increase of radiation loss and acts as an obstacle to miniaturization of the ring resonator sensor as a result.